The invention relates to the general field of magnetic disk systems with particular reference to GMR based read heads.
Read-write heads for magnetic disk systems have undergone substantial development during the last few years. In particular, older systems in which a single device was used for both reading and writing, have given way to configurations in which the two functions are performed by different structures. An example of such a read-write head is schematically illustrated in FIG. 1. The magnetic field that xe2x80x98writesxe2x80x99 a bit at the surface of recording medium 15 is generated by a flat coil, two of whose windings 14 can be seen in the figure. The magnetic flux generated by the flat coil is concentrated within pole pieces 12 and 13 which, while being connected at a point beyond the top edge of the figure, are separated by small gap 16. Thus, most of the magnetic flux generated by the flat coil passes across this gap with fringing fields extending out for a short distance where the field is still powerful enough to magnetize a small portion of recoding medium 15.
The present invention is directed towards the design of read element 20 which can be seen to be a thin slice of material located between magnetic shields 11 and 12 (12 doing double duty as a pole piece, as just discussed). The principle governing the operation of read sensor 20 is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance). In particular, most magnetic materials exhibit anisotropic behavior in that they have a preferred direction along which they are most easily magnetized (known as the easy axis). The magneto-resistance effect manifests itself as an increase in resistivity when the material is magnetized in a direction perpendicular to the easy axis, said increase being reduced to zero when magnetization is along the easy axis. Thus, any magnetic field that changes the direction of magnetization in a magneto-resistive material can be detected as a change in resistance.
It is now known that the magneto-resistance effect can be significantly increased by means of a structure known as a spin valve. The resulting increase (known as Giant magneto-resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of the solid as a whole.
The key elements of a spin valve structure are shown in FIG. 2. In addition to a seed layer 22 on a substrate 21 and a topmost cap layer 27, these key elements are two magnetic layers 23 and 25, separated by a non-magnetic layer 24. The thickness of layer 24 is chosen so that layers 23 and 25 are sufficiently far apart for exchange effects to be negligible (the layers do not influence each other""s magnetic behavior at the atomic level) but are close enough to be within the mean free path of conduction electrons in the material. If, now, layers 23 and 25 are magnetized in opposite directions and a current is passed though them along the direction of magnetization (such as direction 28 in the figure), half the electrons in each layer will be subject to increased scattering while half will be unaffected (to a first approximation). Furthermore, only the unaffected electrons will have mean free paths long enough for them to have a high probability of crossing over from 23 to 25 (or vice versa). However, once these electron xe2x80x98switch sidesxe2x80x99, they are immediately subject to increased scattering, thereby becoming unlikely to return to their original side, the overall result being a significant increase in the resistance of the entire structure.
In order to make use of the GMR effect, the direction of magnetization of one the layers 23 and 25 is permanently fixed, or pinned. In FIG. 2 it is layer 25 that is pinned. Pinning is achieved by first magnetizing the layer (by depositing and/or annealing it in the presence of a magnetic field) and then permanently maintaining the magnetization by over coating with a layer of antiferromagnetic material, or AFM, (layer 26 in the figure). Layer 23, by contrast, is a xe2x80x9cfree layerxe2x80x9d whose direction of magnetization can be readily changed by an external field (such as that associated with a bit at the surface 15 of a magnetic disk).
The structure shown in FIG. 2 is referred to as a top spin valve because the pinned layer is at the top. It is also possible to form a xe2x80x98bottom spin valvexe2x80x99 structure where the pinned layer is deposited first (immediately after the seed and pinning layers). In that case the cap layer would, of course, be over the free layer. The present invention is further directed to a type of spin valve that we refer to as a xe2x80x9cspecular spin valvexe2x80x9d because the conduction electrons are specularly reflected at the NiCr/NiFe interface. We have found, however, that specular spin valves as known to the prior art, for example:
NiCr55/NiFe65/CoFe10/Cu24/CoFe20/MnPt200/NiCr50 (where the numbers represent layer thicknesses in Angstroms and CoFe20 is the pinned layer)
while exhibiting almost twice the GMR effect of non-specular spin valve structures, had lower production yields due to ESD (electrostatic discharge) damage as well as higher scatter in signal amplitude and greater sensitivity to temperature. Thus there exists a need for a specular spin valve that is more robust than those currently known to the art.
A routine search of the prior art was conducted. While several references to various laminated structures within spin valves were encountered, none of these deal with specular spin valves and how to make them more robust. Several of the references found were, however, of interest. For example, Fontana, Jr. et al. (U.S. Pat. No. 5,701,223) forms a pinned layer by strongly coupling two ferromagnetic films in an antiferromagnetic configuration i.e. the two films have a relative antiparallel orientation. The magnetic moments of the two ferromagnetic layers are required to be almost the same. Under these conditions, a pinning layer of nickel oxide may be used and this has the advantage that a cap layer is no longer needed.
Gill (U.S. Pat. No. 5,898,549) forms a pinned layer from three separate pinned layers. The first of these is formed on the pinning layer and, together with the second pinned layer is formed of a high resistivity material such as NiFeCr. They are separated by an anti-parallel coupling layer. The third pinned layer is of low resistivity material such as cobalt.
In U.S. Pat. No. 5,920,446, Gill describes a laminated free layer formed from two ferromagnetic layers separated by a non-magnetic, conducting spacer layer. A key feature is that the two outer layers of the laminate are coupled in an anti-parallel configuration. This arrangement allows the device to operate without a pinned (or pinning) layer.
It has been an object of the present invention to provide a specular spin valve structure that has greater thermal stability of the pinned layer at high reverse fields than specular spin valves of the prior art.
Another object of the invention has been to improve production yields by reducing losses due to ESD damage.
A further object of the invention has been that said spin valve structures exhibit lower scatter in signal amplitudes than specular spin valves of the prior art.
A still further object of the invention has been to provide a process for manufacturing said spin valve structure.
These objects have been achieved by a using a modified pinned layer that is a laminate of three layersxe2x80x94a layer nickel-chromium, between about 3 and 4 Angstroms thick, sandwiched between two layers of cobalt-iron. A key requirement is that the cobalt-iron layer closest to the copper separation layer must be about twice as thick as the other cobalt-iron layer. A process for manufacturing this structure is also disclosed.